Breeding of fruity vegetables like cucumber aims at the production of commercial varieties optimally adapted to a professional production environment in order to produce marketable products. Many characteristics need to be taken into account during selection which relate to both input and output traits. One of the most important traits in this respect relates to post harvest quality, in particular to the shelf life of fruits. The avoidance of post harvest deterioration of fruits is an important element that can significantly contribute to an economically more efficient arrangement of the whole production chain.
From the moment of harvest of the cucumber crop until the moment of consumption, the produce is exposed to different exogenous factors contributing to product deterioration. Such factors can be wounding during harvesting and processing, darkness and nutrient deficiency during storage and ethylene during processing, transport and storage. These factors strongly stimulate the post harvest disorders which can become apparent as yellowing or loss of firmness of the fruits. As a consequence of these effects the product becomes much less attractive and thereby unmarketable.
In order to counter the deterioration effects, many post harvest measurements can be taken which reduce these effects. For example, one can store the harvested cucumber fruits at relatively low temperatures of around 10° C. to retard senescence. In addition, logistic measurements may be implemented which reduce the transportation time required from the greenhouse or field to the consumer or which prevents the cucumber fruits to be stored in the vicinity of an ethylene source. Furthermore chemical treatments such as with 1-methylcyclopropene (1-MCP) or others may be applied to prevent the post harvest deterioration although food safety and consumer acceptance obviously become an issue.
Many of the post harvest measurements are successful to some extent but there is certainly room for improvement. Moreover, costs involved may be substantial, which is another reason to explore alternatives which reduce the need to apply post harvest treatments. Although knowledge of the physiology of post harvest deterioration is limited, senescence seems to play an important role in this respect.
Senescence is a naturally occurring, developmental process at the end of a life cycle of a plant or plant organ during which metabolism is reprogrammed in order to remobilize resources into reproductive structures like seeds. Yellowing of green leaves or immature green fruits is the most visible symptom of senescence. These are a consequence of chlorophyll breakdown during a relatively late stage of senescence.
Ethylene is an important plant hormone generally known to stimulate physiological processes related to senescence once a leaf or fruit is receptive. In cucumber this stimulation becomes apparent through the yellowing and reduced firmness of harvested fruits. Loss of fruit firmness relates to the enzymatic break down of cell wall constituents like pectine, cellulose and hemicellulose. As a consequence the tissue integrity of the fruits is gradually lost which leads to a strong increase of pathogen susceptibility which usually manifests by fungal growth at the fruit surface.
Although fruits of cucumber are known to produce only low amounts of ethylene, especially during the immature phase, the fruits are highly sensitive towards this plant hormone. Therefore, physiological disorders associated with ethylene sensitivity which reduce the post harvest quality of cucumber can be caused by endogenous ethylene synthesis but external sources of ethylene can also be very important in this respect. Exposure to such external sources can occur during harvesting, processing and storage of the produce.
For example, when cucumber fruits are transported or stored in the vicinity of ethylene producing fruits such as apples, pears, bananas or peaches severe deterioration may occur. Furthermore, when cucumber is processed and used as packaged fresh-cut product or in fresh-cut mixtures there may be limitations with respect to the ingredients which can be used due to ethylene release by one or more of the ingredients.
Although ethylene is the most important plant hormone known to stimulate senescence, other hormones like jasmonate may also contribute to this process.
However, in addition to playing a role in senescence, ethylene is also known to be involved in many other physiological processes. In cucumber ethylene plays an important role in the determination of sex expression of the flowers. In general, ethylene treatment of cucumber flower buds enhances femaleness. On the other hand when the sensitivity of female flower buds towards ethylene is reduced for example through the application of silver ions which bind the ethylene binding site of the ethylene receptor protein, the flowers will change their developmental program to enhance the development of male organs. A change in sex expression of the flowers is highly undesirable and must be avoided. It is generally thought that in cucumbers lowering the sensitivity to ethylene is not a suitable way of delaying post-harvest deterioration to increase shelf life.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.